The present invention relates generally to power supplies and particularly to power supply output control.
Integrated circuits typically have varying requirements for power supply voltages. Various integrated circuits may require different power supply voltage levels and different ramp-up times for the power supply voltages.
One method for controlling the ramp-up time for a power supply is to couple a pass transistor to the output of the power supply. The transistor turns on slowly, feeding current to the load at a reduced rate, while the power supply ramps up at a much quicker rate. Once the power supply output is at its maximum, the transistor appears as a short circuit to the power supply. However, transistors typically do not make a perfect short and, therefore, waste some power supply energy in the form of heat.
The heat dissipation of the pass transistor may be reduced by using a transistor that has a very low xe2x80x9conxe2x80x9d resistance. These transistors are typically very expensive, thus adding to the cost of the power supply. There is a resulting need for a power supply that can economically control the ramp-up time of its output voltage without wasting power supply energy.
Embodiments of the present invention provide a slow output voltage ramp-up by controlling the output voltage ramp-up on the primary side of the power supply""s transformer. The power supply comprises a feedback voltage that is coupled to the output voltage of the power supply. The feedback voltage is proportional to the output voltage.
A controller has a current sense input, a voltage feedback input, and a control output. The voltage feedback input is coupled to the feedback voltage and the controller adjusts a current sense threshold in response to the feedback voltage level. A control circuit apparatus is coupled to the current sense input of the controller. The control circuit apparatus comprises an offset voltage generator and a timer controlled switch coupled to the offset voltage generator. The timer-controlled switch applies the offset voltage to the current sense input during a predetermined time interval set by the current sense threshold. This turns the control output on during the predetermined time interval. An output control switch is coupled to the control output. The switch turns on when the control output is on.